Sound muffler



W. P. MASON Aug. 30, 1932.

SOUND MUFFLER Filed June 14. 1929 2 Sheets-Sheet 1 Fla. 3

FIG. 2

INVENTOR W MASON BY%W ATTORNEY Patented Augao, 1932 UNETED STATES PATENT OFFICE 'WARREN P. MASON, OF EAST ORANGE, NEW JERSEY, ASSIGNOR TO I B ELL TELEPHONE LABORATORIES, INCORPORATED, OF NEW YORK, N. Y., A. CORPORATION OF NEW YORK sormn MUFFLER Application filed June 14,

This invention relates to noise suppressing devices and more particularly to exhaust mufiiers for internal combustion engines. It has for its principal objects increasing the degree of noise suppression and diminishing 'struction whichmay be used'in the mufller the effective back pressure against the flow of the exhaust gases. Another object is to diminish the weightof exhaust mufflers-for a prescribed degree of noise suppression and back pressure.

These objects are accomplished by making use of the principle of wave interference as in acoustic wave filters. The component gas vibrations which make up the periodic train of exhaust impulses are operated on .insuch manner that all of the high frequency vibrations, which give the exhaust noise its sharp quality, are strongly attenuated in the muffier, while the low frequency components pass through the exhaust passage without obstruction. Free passage of the low frequency components is essential to the prevention of back pressure, but, by limiting the low frequency components that can be passed freely to the range below about 200 cycles per second, back pressure is prevented and the exhaust noise is reduced to a soft pulse of slight "audibility.

In its general form, the invention comprises a main gas conducting channel having apertures disposed at intervals along its length, opening into enclosed chambers. An important feature of the invention is that the side chambers are of graded sizes, this graduation being essential for the satisfactory suppression of all of the higher frequency, or noise creating, vibrations. Another feature of the invention is that the exhaust passage contains no bafiles, which if present, would tend to impede the free passage of the. low frequency vibrations andthereby create objectionable back pressure. Due tothe absence of baffles, it is possible to construct a mufiler which, in respect to the degree of its noise 1929. Serial No. 370,816.

suppressing properties, is extremely light in weight.

In thedrawings, Fig. 1 illustrates in section one form of the invention;

Fig. 2 illustrates in section a modifiedconshown in Fig. 1;

Fig. 3 shows a side'vie'w, in section, of another modified construction;

Fig. 4 illustrates in section a variation of the muffler shown in Fig. 1;

Fig. 5 shows another variation;

Fig. 6 shows a side sectional view of a form of mufller in which heat fromhot gases can be more readily radiated; and

a Fig. 7 illustrates typical sound attenuation characteristics of mufflers. embodying the invention.

In the case of a sound mufller applied to a .gas engine the exhaust gases from the engine are discharged into the mufiler in a. series of sharp pufis or explosions which follow each other in a regular succession. The noise that would be produced by the explosions is not of a recognizably tonal character, but nevertheless it may be regarded as being the resultant of a multiplicity of pure tones having certain definite phase andamplitude relationships. Moreover, so long as the engine is running at an approximately steady speed, the explosions follow each other at regular intervals which results in a harmonic relation being established between the frequencies of the component tones, and in a relatively simple tone composition of the exhaust noise. If the high frequency components are removed the noise loses its sharp quality, and if all, of the component tones down to about 200 c. p. s. or lower, are removed, only a rather soft pulsing noise remains. If the pulses of frequencies up to about 200 c. p. s.. are permitted to pass freely, practically no back'pressure is set up, which might interfere with the operation of the The mufiier in Fig. 1 comprises a cylinder 10 divided into chambers 11, 12, 13, 14, '15 and 16 by means of transverse partitions-17; Communicating passages are provided between the chambers by tubular'elements 18 which pass through the centers of the parti-- tions and provide a path for the gas flow.

Similar tubular elements 19, extending through the end partition provide means for introducing and ejecting the exhaust gas The mufiler in the drawings is composed of three stages, chambers ,11 and 12 constituting one stage,'-1 3 and'14 a'second stage, and 15 and 16 a third stage. The chambers and tubular gas conducting elements constituting a stage are of uniform size, but the dimensions vary in the different sta es.- Referring to the first stage on the left, or example, chamb'er's 11 and 12 are of equal size and so alsoare the lengths Z that the tubular elements extend into each of the chambers. Similar- .-ly,- in the next stage, chambers 13 and 14 are equal, and likewise the associated pipe lengths; but. the component parts in this stage are larger than in the first stage. In the drawings, each of the stages is shown divided into two chambers, although a greater number ofchambers may be provided if desired, and in-certain cases itmay be found suflicient' to have a single chamber in each stage. The mufiling properties, however, are improved by increasing thenumber of chambers per stage, and also by increasing the number ofstages.

The construction may be of any material capable of withstanding heat and vibration.

The partsm'ay be made of sheet steel welded together. The pipe extendin from one end in each .stage, however, is not complete. There.'.are certain regions in'thehigher freof the mufiler may be attache to the exhaust pipe of'the engine by any suitable means,-

such as threads or a'flange.

' The muffler operates as follows: Each of the stages acts as a sound wave filter which permits the passageof low tones but suppresses tones ,of a frequency higher'than a I the noise suppression range. An examinacertain value determined by its dimensions. The suppression of the higher pitched. tones quency range in which the tones are transmitted almost as freely as the low pitched tones. These ranges have been found-to oc-.

cu'rat regular harmonic intervals, and their occurrence can be predicted from the dimensions of the air chambers and passages. -By varying the dimensions of the air chambers in the successive stages the transmission r'angesof one stage can be made to an within the suppression ranges of the otherstages, with the result' that all of the high frequency noise components are suppressed, and only the 'nearly inaudible frequencies are transmitted. By the combination of properlyproportional stages the noise suppressioncan mg of the bands.

be efiected' with a mufiier of minimum size and-at a substantial saving of weight.

The propagation of sound waves through one of the stages is expressed by the formula:

CoshP cos cos S e 21 Li l 2 i- L a sfisg m u Smr v where P=the propagation constant perchamber of the stage;

'2Z= the length of the central conduit in the chamber;

L=length of. the chamber of gas conduction;

S ='cross-sect1onal area of the central conin the direction .duit;

S =cross-sectional area of the chamber; v=velocity of sound propagation; w=21r time'sthe frequency of sound waves.-

The propagation constant P is, in general, a

complex quantity and can-beexpressed as:

P=A+jB (2) condition, which is also defined by the relation Cosh P :l; 1, A the vibrations are transmitted without sub-' stantial attenuation, but are changed in phase as they "traverse the section. The second condition, which corresponds to (Josh P i1,

represents the frequency ranges in which the v1 rations are strongly attenuated, that is,

tion of Equation (1) shows that there is. a band of free transmission beginningat zero frequency, and also an infinite number of??- cies -occurring in harmonic order. In gen-- eral, the 'attenuationbands are considerabl wider than the transmission bands which separate them, and their widths'are dependent upon the areas S and S the larger the quantity I 1 2 the narrower become the harmonically related transmission bands. .The location of the transmission bands is dependent upon the transmission bands centeredaligut frequen- 5 I pipe and chamber dimensions, Z and L; the I greater these lengths the closer is the spac- To operate effectively a mufiler should have an initial transmitting range from zero to about 100 or 200 cycles per second, followed by an attenuation range embracing all of the higher audible frequencies. In a stage complurality of stages, each stage having expan-' sion chambers of a different size from the other stages. The lengths of the connecting passages may also be varied from stage to stage for the same purpose. these chambers are preferably so proportioned with respect to each other that the central frequencies of the transmitting ranges of the one stage bear incommensurate ratios to those of the other stages, thereby causing the higher frequency transmission bands to be eliminated.

Computed attenuation characteristics of a typical muflier in accordance with the invention are shown in Fig. 7. Curve a shows the theoretical attenuation of one stage comprising a single chamber whose dimensions are:

The regions of free transmission are indicated by the heavy lines on the axis of abscissae. It is seen that this stage, or section, has several transmitting bands in the audible frequency range. To eliminate these transmission bands, a stage having the chamber dimensions,

may be added. The theoretical attenuation of one such chamber is shown by the dotted curve 6. Curve 0 represents a typical attenuation characteristic such as would actually be obtained in a mufiier comprising a chamber corresponding to curve a and a chamber corresponding to curve 6. This mufller transmits frequencies from zero to about 220 c. p. s. Curve d represents a typical attenuation characteristic of the mufiler corresponding to curve a to which a chamber with dimensions,

1 5' 5' 1:4, L- 16" and 5 +3; -7,

transmission is the decibel, which is a logarithmic measure of the wave power attenuation. A treatment of the decibel is given by W2 H. Martin in the Journal of the Ameri- The sizes of can Institute of Electrical Engineers, page 223, March 1929. I

Fig. 2 illustrates a modified construction of a mufiier stage in which back pressure arising from the expansion of the exhaust gases is more com letely eliminated. Back pressure from this cause is 'due mainly to the creation of eddies at the openings into the expansion chambers and the modification illustrated reduces this back pressure by preventing the creation of such eddies. The construction in Fig. 2 obviates the formation of eddies by the use of a series of tapered tubes 20 for the central circuit, the convergence being inthe direction of the gas flow. This construction produces a contraction in the diameter of the jet as it passes from one chamber to the next, the subsequent expansion of the jet not being great enough to make its diameter greater than the mouth of the next tube.

Fig. 3 is another modification of a muflier stage in which the effect of eddies is similarly reduced. In this construction the gas channel, instead of comprising short lengths of pipe through the partitions separating the chambers comprises a single pipe 30 extending entirely through the chambers. The por tions of the pipe in the centers of the chambers are drilled with numerous holes 31 which serve for communication with the chambers. The ratio of undrilled pipe surface to the area of the holes in the drilled portion of the pipe may conveniently be made unity. It is desirable that the ratio of diameter of holes to the thickness of the pipe should not be greater than 5, to prevent a whistling which mightotherwise occur. In the design of stages of this type, the length 21 is taken as the length of the gas pipe in each chamber which is left undrilled.

Fig. 4 shows a muffler in which the ratio as well as the axial lengths of the chambers is varied from stage to stage. The mufiier comprises a cylinder 10 partioned into three stages of two chambers each, by means of partitions 17 through which extend gas conducting elements of the tapered type shown in Fig. 2. The particular feature of this muffler is that the cross-section of the gas jet is reduced in passing from one stage to the next. In the first stage, comprising chambers 11 and 12, the cross-section of the jet is approximately that of the end pipe 19. But the jet passing through chambers 13 and 14, which constitute the second stage, has the cross-section of the small end of the tapered gas conducting element 41, which tapers to a smaller cross-section than that of pipe 19. In a similar manner the tapered element 42 reduces the jet in the third stage to a still smaller cross-section.

' tin The reduction in the size of the gas channel oil of air-or steam or the exhaust noise from increases thequantity steam engines They. mayalso be .used, as

. S ventilators where it is desired to admit a free flow of air into-a room, and at the same 1 S2 time prevent outside noise from entering.

which has the efl'ect'of widening the attenuation bands. Since, in passing through'the mufiler, the gas cools and contracts, it is generally feasible to reduce, somewhat the si'ze of-the' gas pipesin 'succ'eeding'stages and'thus obtain the advantage of narrower transmitig. 5 illustrates a mufiler in which the ratio is adjusted by an alternative-method; In

' this-construction the cross-section'of the'gas channel iskept constant, whi1e that of the outershell is increased from stage-to stage.

. :The muflier comprises three closed-ended cytitions 17 The gas conducting elements 18 extending through the centersof the partitions are uniform in cross-section.

.cordance with the invention,- in 'whichthe .Figs. 6 and 6A illustrate, respectively, side and end views in section of a muflier'in accentral gas passage is exposed to the sur rounding air so that heat will be more readily radiated.' The muflie-r comprises a gas pipe "60, on'opposite sides ofwhich' are located shells 61 and 62, of equal size, in which the' expansion chambers are formed. Each shell may conveniently be a length of pipe having a segment cut out longitudinally. The edgesleft by the removal, of the'segmentare then fractional with each of said chambers.

- What is claimed is: v y 1. A sound mufiler comprising-an exhaust pipe forming a continuous exhaust passage,

a tubular member disposed parallel to said pipe; and attachedthereto along its length, and a'plurality of transverse partitions subdividing said tubular member into a'pluralitv of chambers equal in pairs, the axiallen'gt -'of the chambers of the several pairs increasing progressively from one end of the tubular .member to-the other, the exhaust pipe having lateral apertures communicating with each of said chainbersya'nd the said tubular member comprisinga tube segment enclosing a portion of the periphery of the exhaust pipe,

2. A sound muflier comprising an exhaust pipe forming a continuous exhaust passage,

pipe on opposite sides thereof and attached thereto along its length, and a plurality of .tubularmembers disposed parallel to said "n transversepartitions subdividing said tubular members into chambers equal in pairs, the

axial length of the chambers of the several pairsprogressively increasing from one end of the'tubular member to the other and corresponding tothe lengths. of the chambers on the opposite side of said exhaust pipe,.and said 'ex-haus't pi-p e having apertures at points along its length to provide gas communication In witness whereof, -I- hereunto subscribe my name this 13th day of June, .1929.

WARREN P.- MASON.

bent back and welded to the circumference of the central pipe 60. Each shell is partitioned into the chambers bythe partitions 63, and the I chambers in the two shells which are oppo P sitely located are made equal 'in.siz'e,'that is,

the expansion chambers are arranged in pairs,

.Numerous -small holes- 64,- communicating with theexpansion chambers, are provided in the--pip'e60,' similar tothose shown in Fig. 3.

-' In designing a muffler of this type, the

quantity 2 tional area of a pair of expansionchamber-s.- 'Ihe' length 21 as n Fig. 3,1s'the length of theis taken as the total cross-secinc central pipe in" each chamber which is 'free from holes.

1 It can be seen'from the fi '1 re that a large part ofthe gas conducting pipe is exposed -'to' the air whereby heat cangbe readily radiated; Mufiers of the type described can be used for a variety'ofpurposes besides the suppressi'on of exhaust noises frominternal combus-- tion'enginesfior example, they may beus ed' to silence the hissing-noise from the blowing 

